The invention relates to a method of opening a closure disc arranged on a pressurized container as well as to a compressed gas source for activating a vehicle occupant restraint system.
Already known from DE 195 24 094 A1 is a burst disc on a pressurized container for a vehicle occupant restraint system which may be perforated by an electric arc or weakened at least to the extent that it bursts when exposed to the internal pressure of the compressed gas container. The arc discharge may be struck between the surface of the burst disc and an electrode arranged spaced away from this surface and fed by a high-voltage source. In actual practice, however, it has been found out that a reproducible opening response cannot be attained by this method, the burst diaphragm either being perforated only locally or bursting in an uncontrolled manner. For satisfactory functioning of a restraint system, especially an inflatable protective cushion, it is mandatory, however, that a gas exit port is exposed on the compressed gas container with a reproducible flow cross-section. Since this requirement fails to be satisfied by the known method it has not achieved any significance in actual practice.
By means of the invention a method of opening a closure disc arranged on a pressurized container is provided with which a gas exit port having a reproducibly determined flow cross-section may be exposed in the closure disc of the pressurized container by applying an arc discharge. In accordance with the invention a linear zone of interaction is delimited between an electrode and the surface area of the closure disc by means of an electrical insulation, the arc discharge being initiated in a punctiform manner and then continued over the length of the linear zone of interaction. The closure disc is weakened along the linear zone of interaction between the two ends thereof by the arc discharge, at least to such an extent that under the effect of the internal pressure of the pressurized container and starting from both ends of the weakened zone of interaction, a strip of material is peeled and bulged open from the closure disc in a direction transverse to the linear zone of interaction. The width of the material stripxe2x80x94and thus that of the gas exit port formed in the closure discxe2x80x94is defined by the extent of the linear zone of interaction. It has been found out that between the linear end face of the electrode and that of the surface of the closure disc opposite thereto, the arc discharge initially occurs in a punctiform manner before the electric arc tends to spread along the predefined zone of interaction due to the high current density. The electric arc perforates the closure disc initially in a punctiform manner and is then propagated laterally under the effect of escaping gas so that the electric arc continues to progress on both sides of the punctiform perforation, thus perforating the closure disc along the full zone of interaction or weakening it at least to a sufficient degree so that under the effect of the internal pressure of the pressurized container peeling occurs, commencing along the linear zone of interaction. At the ends of the zone of interaction shear stress within the material of the closure disc results in a shear effect as a result of which a strip of material is peeled from the closure disc between the ends of the zone of interaction. The length of the strip of material thus peeled is substantially defined by the rim of the closure disc. Accordingly, a gas exit port is exposed in the closure disc, the width of which is defined by the width of the linear zone of interaction and the length of which is determined on the one hand by the localization of the zone of interaction and, on the other, by the rim of the closure disc, the gas exit port thus formed, therefore, being reproducible in shape and size.
As compared to conventional methods, the method in accordance with the invention offers the advantages of being highly practical and safe. unlike pyrotechnic-based methods it has the advantage that no toxic substances and combustion products are formed whilst eliminating the need to handle and stock explosive materials. As compared to all known methods it has the advantage of an extremely fast opening response, practical tests having demonstrated that a period of only slightly more than 100 ms is sufficient for the opening response. Conventional methods are slower by a factor 10 and more. Fast activation is a salient criterion especially in the case of restraint systems for protection against side impact.
It has furthermore been found out that a surprisingly low amount of electrical energy is sufficient for successfully implementing the method.
Tests have indicated it to be particularly favorable to provide a discharge path very small in length, this being achieved by the end face of the electrode being directly contacting the surface area of the closure disc so that the arc discharge occurs at an extremely high amperage which may amount to a few thousand amperes, whilst operation is permitted with a low discharge voltage in the range of but 100 volts or less. It has been found out that increasing the voltage achieves no significant improvement in the opening response. Employing a low discharge energy has in addition the advantage that it may be stored in a capacitor of acceptable size, good results having been attained with a capacitor having a capacitance of approx. 4 to 8 mF. Depending on the strength of the voltage employed the capacitance of the capacitor may be in the range 1 to 10 mF, corresponding to a voltage range of approx. 50 to approx. 200 volts.
It is also the object of the invention to provide a compressed gas source for activating a vehicle occupant restraint system in which the disc closing off the pressurized container is opened by the method in accordance with the invention to activate the restraint system. The salient feature of the compressed gas source in accordance with the invention is an electrical insulation defining a linear zone of interaction between the electrode and the surface area of the closure disc.